A cellular wireless communication system has been improved from Universal Mobile Telecommunication System (UMTS) to Long Term Evolution (LTE). LTE applies Orthogonal Frequency Division Multiplexing (OFDM) and Single Carrier-Frequency Division Multiple Access (SC-FDMA) to a downlink wireless access technique and an uplink wireless access technique, respectively, to achieve high-speed wireless packet communication having a downlink peak transmitting rate of 100 Mb/s or more and an uplink peak transmitting rate of 50 Mb/s or more.
OFDM, which is the downlink access technique of LTE, allocates a wireless resource both in the time direction and the frequency direction to each user, in other words, mapping a physical channel to a resource element. Here, mapping of physical channels to adjacent cells will now be described with reference to FIGS. 1A and 1B.
FIGS. 1A and 1B depict resource blocks (RBs), which is a unit of a wireless resource allocated to each user in LTE, and illustrates an example of the respective resource blocks of two adjacent cells (FIG. 1A depicts cell 1; and FIG. 1B depicts cell 2). The abscissas and the ordinates of the resource blocks in FIGS. 1A and 1B respectively represent frequency and time in units of subframes. A resource block is a two-dimensional wireless resource unit of a frequency and a time segmented by 12 subcarriers (12 SC) and 14 OFDM symbols. A region surrounded by a single subcarrier (1 SC) and a single OFDM symbol is called a resource element (RE).
A control signal (represented by “C” in units of resource elements in FIGS. 1A and 1B) can be variably determined by the base station of each cell in a region of first through third OFDM symbols from the top. Thereby, the minimum wireless resource for transmitting a control signal can be reserved. In the example of FIGS. 1A and 1B, control signals are mapped in the first through the third leading OFDM symbols in the cell 1 and in the first through the second OFDM symbols in the cell 2.
Control signals are classified into the following three types:
(1) PCFICH (Physical Control Format Indicator Channel)
a signal that notifies the number of OFDM symbols (Control format indicator: CFI) for a control signal;
(2) PHICH (Physical Hybrid ARQ Indicator Channel)
a signal that notifies ACK/NACK information related to an unlink shared channel (PUSCH); and
(3) PDCCH (Physical Downlink Control Channel)
a signal that notifies allocation information of downlink and uplink resources.
Reference signals (Cell-specific RS; in the following description, appropriately abbreviated to CRSs) peculiar to each cell are mapped equally in the time and frequency directions of a resource block. Such reference signals are used for detecting an FFT timing and for estimating a channel of a mobile terminal. Arrangement of a reference signal is shifted in the frequency direction, depending on the identification (cell ID) peculiar to each cell. In the example of FIGS. 1A and 1B, the entire arrangement of reference signals in the cell 2 is shifted by one subcarrier from that in the cell 1. This shifting avoids interference between reference signals in the adjacent cells.
FIGS. 1A and 1B are an arrangement of reference signals conforming to the MIMO (Multiple Input Multiple Output) scheme. In the drawing, “R0”, “R1”, “R2”, and “R3” represent resource elements in which reference signals corresponding to four antenna ports (Antenna ports 0-3), respectively, are mapped.
Downlink shared channels (PDSCHs) are mapped into the remaining resource elements except for resource elements in which controls signals or reference signals are mapped. Consequently, the remaining resource elements are used for data transmission to mobile terminals. In other words, PDSCHs, i.e., data, are mapped in the resource elements not marked with “C” or “R” (“R0”, “R1”, “R2”, and “R3”) in FIGS. 1A and 1B.
For higher-speed communication, the 3rd Generation Partnership Project (3GPP), an international standardization organization, is developing a downlink technique in a wireless communication system LTE-A (LTE-Advanced) based on LTE which technique is called Coordinated Multi-Point (CoMP) transmission scheme. The CoMP transmission scheme causes multiple base stations to transmit a PDSCH to a particular mobile terminal in coordination with each other. Current agreements of the CoMP transmission scheme are described in the specification 3GPP TR 36.814.
Joint transmission (CoMP JT) is known as one of the embodiments of the CoMP transmission scheme. The basic concept of CoMP JT will now be described with reference to FIG. 2.
In normal cellular communication, a mobile terminal receives an interference signal from a second cell adjacent to a first cell that the mobile terminal is connecting. For this reason, the communication has a problem of large deterioration of receiving characteristic of a mobile terminal especially positioning at a boundary of a cell. In contrast, CoMP JT causes the base stations of multiple cells (cell 1, cell 2) to transmit PDSCHs based on the same data to a particular mobile terminal UE 1, as illustrated in FIG. 2. With this configuration, since a mobile terminal receives a desired signal containing the same data from a base station that the mobile terminal is connecting and also from a contiguous base station, the above problem of deterioration in receiving characteristic of a mobile terminal at a cell boundary can be solved.
The receiving characteristic of a mobile terminal at a cell boundary can be further improved by transmitting a PDSCH after being subjected to an encoding scheme known as Precoding (a kind of transmitting beam forming) at a wireless mobile station. LTE applies the Codebook scheme to the Precoding (see the specification 3GPP TS 36.211, and 36.213). Specifically, multiple candidates for a Precoding matrix to be multiplied by a transmitting signal are prepared commonly to a transmitter station and a receiver station. The receiver station selects an optimum candidate of the Precoding matrix using the result of estimating a wireless channel state, and feeds the identifier (Precoding Matrix Indicator: PMI) of the selected Precoding matrix to the transmitter station. The transmitter station multiplies the Precoding matrix corresponding to the feedback PMI by a transmitting signal and then transmits the processed transmitting signal to the receiver station.    Non-Patent Literature 1: 3GPP TS 36.211 V8.8.1 (2009-09), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)    Non-Patent Literature 2: 3GPP TS 36.213 V8.8.0 (2009-09), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures (Release 8)    Non-Patent Literature 3: 3GPP TR 36.814 V1.6.0 (2010-1), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Further Advancement for E-UTRA; Physical Layer Aspects (Release 9)
Here, on the premise of the mapping of physical channels of resource blocks of FIGS. 1A and 1B, it is assumed that multiple base station transmit PDSCHs based on the same data to a particular mobile terminal through the CoMP JT scheme. Specifically, CoMP JT is applied to between the base stations of the cell 1 and the cell 2 depicted in FIGS. 1A and 1B. FIG. 3 illustrates problems of this case. FIGS. 3A and 3B correspond to FIGS. 1A and 1B, respectively. In FIGS. 3A and 3B, a resource element in which a reference signal is mapped is represented by “R” and a resource element in which a PDSCH to be transmitted in the CoMP JT scheme is not mapped is represented by “X”.
In FIGS. 3A and 3B, a PDSCH based on the same data can be arranged to a resource element in which the base stations of the cell 1 and the cell 2 can commonly map a PDSCH. Unfortunately, a resource element in which either cell maps a signal different from a PDSCH, that is a control signal (“C”) or a reference signal (“R”), does not achieve the CoMP JT scheme. For example, since reference signals “R” are mapped in the resource elements in the cell 2 corresponding to those marked with “X” in the cell 1, the CoMP JT is not achieved in the above resource elements. Since control signals are mapped in the third OFDM symbol in the resource block of the cell 1, the corresponding OFDM symbol of the cell 2 does not achieve the CoMP scheme.
As the above, since multiple base stations do not transmit a PDSCH based on the same data through a wireless resource in which either cell maps a signal different from a PDSCH, i.e., a control signal or a reference signal, data transmission efficiency is poor.